A large body of evidence exists implicating the thymus and thymus-derived (T) cells in regulation of hemopoiesis. Recent data from in vitro culture of bone marrow cells in the presence of thymocytes at various T:BM ratios suggest that T lymphocytes may be effective at suppression as well as augmentation of CFU-E, BFU-E, and CFU-GM. We propose to test separated T lymphocyte subpopulations of putative helpers (Ly 1 + 2/3 - ) and suppressors (Ly 1 - 2/3 + ) for their ability to affect CFU-E and other precusors of blood formation in order to establish whether two distinct cell types are involved in regulation of myelopoiesis. Lymphocytes will be selected positively by panning and negatively by cytotoxicity using antisera (either isoimmune or rat-mouse hybridoma-produced) specific for individual Ly receptors. These subpopulations will also be tested for their ability to modulate in vitro growth of lymphocyte progenitors (pre-T and pre-B) to probe the means by which T cells might regulate not only functional lymphocytes but their immature marrow precursors as well. We will test the validity of in vitro results for the intact animal by in vivo experiments to determine the likelihood that mature T lymphocytes engage in negative feedback control. Thymectomy, antithymocyte serum, and localized engage in negative feedback control. Thymectomy, antithymocyte serum, and localized irradiation will be used to deplete, and intravenous infusion of T cells and transplantation of auxiliary thymic tissue of enrich circulating lymphocyte numbers. Information to be gained from these studies will contribute to our knowledge of regulation of myelopoietic and lymphopoietic progenitors in the marrow.